There are currently about 262,000 spinal cord injured (SCI) individuals in the United States, with roughly 12,000 new injuries sustained each year at an average age of injury of 40.2 years. Of these, approximately 44% (5300 cases per year) result in paraplegia. One of the most significant impairments resulting from paraplegia is the loss of mobility, particularly given the relatively young age at which such injuries occur. Surveys of users with paraplegia indicate that mobility concerns are among the most prevalent, and that chief among mobility desires is the ability to walk and stand. In addition to impaired mobility, the inability to stand and walk entails severe physiological effects, including muscular atrophy, loss of bone mineral content, frequent skin breakdown problems, increased incidence of urinary tract infection, muscle spasticity, impaired lymphatic and vascular circulation, impaired digestive operation, and reduced respiratory and cardiovascular capacities.
In an effort to restore some degree of legged mobility to individuals with paraplegia, several lower limb orthoses have been developed. The simplest form of passive orthotics are long-leg braces that incorporate a pair of ankle-foot orthoses (AFOs) to provide support at the ankles, which are coupled with leg braces that lock the knee joints in full extension. The hips are typically stabilized by the tension in the ligaments and musculature on the anterior aspect of the pelvis. Since almost all energy for movement is provided by the upper body, these (passive) orthoses require considerable upper body strength and a high level of physical exertion, and provide very slow walking speeds. The hip guidance orthosis (HGO), which is a variation on long-leg braces, incorporates hip joints that rigidly resist hip adduction and abduction, and rigid shoe plates that provide increased center of gravity elevation at toe-off, thus enabling a greater degree of forward progression per stride. Another variation on the long-leg orthosis, the reciprocating gait orthosis (RGO), incorporates a kinematic constraint that links hip flexion of one leg with hip extension of the other, typically by means of a push-pull cable assembly. As with other passive orthoses, the user leans forward against the stability aid while unweighting the swing leg and utilizing gravity to provide hip extension of the stance leg. Since motion of the hip joints is reciprocally coupled through the reciprocating mechanism, the gravity-induced hip extension also provides contralateral hip flexion (of the swing leg), such that the stride length of gait is increased. One variation on the RGO incorporates a hydraulic-circuit-based variable coupling between the left and right hip joints. Experiments with this variation indicate improved hip kinematics with the modulated hydraulic coupling.
In order to decrease the high level of exertion associated with passive orthoses, the use of powered orthoses has been previously investigated, which incorporate actuators and an associated power supply to assist with locomotion. More recently, a powered orthosis was developed by combining three electric motors with an RGO, two of which are located at the knee joints to enable knee flexion and extension during swing, and one of which assists the hip coupling, which in essence assists both stance hip extension and contralateral swing hip flexion. The orthosis was shown to increase gait speed and decrease compensatory motions, relative to walking without powered assistance.
In addition, control methods have been proposed for providing assistive maneuvers (sit-to-stand, stand-to-sit, and walking) to paraplegic individuals with the powered lower limb orthosis HAL, which is an emerging commercial device with six electric motors (i.e., powered sagittal plane hip, knee, and ankle joints). Like the powered lower limb orthosis HAL, two additional emerging commercial devices include the ReWalk™ powered orthosis from Argo Medical Technologies and the eLEGS™ powered orthosis from Berkeley Bionics. Both of these devices were developed specifically for use with paraplegic individuals.